Abstract:

An image forming apparatus includes a storage unit configured to store a
recording medium, an image forming unit configured to form an image on
the recording medium, a paper feed unit configured to supply the
recording medium from the storage unit, a conveying unit configured to
convey the recording medium supplied by the paper feed unit to the image
forming unit, and a control unit configured to control the paper feed
unit and the conveying unit in conveying the recording medium. In
accordance with a conveyance state in which the recording medium supplied
from the storage unit by the paper feed unit is conveyed to the conveying
unit, the control unit controls the conveying speed of the next recording
medium supplied by the paper feed unit.

Claims:

1. An image forming apparatus comprising:a storage unit configured to
store a recording medium;an image forming unit configured to form an
image on the recording medium;a paper feed unit configured to supply the
recording medium from the storage unit;a conveying unit configured to
convey the recording medium supplied by the paper feed unit to the image
forming unit; anda control unit configured to control conveyance of the
recording medium by the paper feed unit and the conveying unit,wherein,
in accordance with a conveyance state in which the recording medium
supplied from the storage unit by the paper feed unit is conveyed to the
conveying unit, the control unit sets a conveying speed of the next
recording medium supplied by the paper feed unit.

2. The image forming apparatus according to claim 1, wherein the
conveyance state includes a time period between a time when the recording
medium is supplied by the paper feed unit and a time when the recording
medium reaches the conveying unit.

3. The image forming apparatus according to claim 1, wherein the control
unit exerts control so that the first recording medium supplied by the
paper feed unit after an input of a print start command is conveyed to
the image forming unit after being temporarily stopped at the conveying
unit, and so that the second and subsequent recording media supplied by
the paper feed unit are conveyed to the image forming unit without being
stopped at the conveying unit.

4. The image forming apparatus according to claim 2, wherein the control
unit sets a conveying speed of the recording medium supplied by the paper
feed unit so that a difference between the time period and a preset
period is within a preset range.

5. The image forming apparatus according to claim 2, further comprising:a
first detecting unit configured to detect the recording medium supplied
by the paper feed unit,wherein the conveying speed of the recording
medium by the paper feed unit is set on the basis of a time period
between a time when the recording medium is supplied by the paper feed
unit and a time when the recording medium is detected by the first
detecting unit.

6. The image forming apparatus according to claim 5, further comprising:a
second detecting unit configured to detect the recording medium after the
recording medium is detected by the first detecting unit and before the
recording medium reaches the conveying unit,wherein the conveying speed
of the recording medium by the paper feed unit and the conveying unit is
set on the basis of a time period between a time when the recording
medium is detected by the first detecting unit and a time when the
recording medium is detected by the second detecting unit.

7. The image forming apparatus according to claim 1,wherein the storage
unit includes a presence/absence sensor configured to detect the presence
or absence of the recording medium, andwherein the control unit
determines whether to stop the recording medium at the conveying unit in
accordance with the detection result of the presence/absence sensor.

8. The image forming apparatus according to claim 1,wherein the image
forming unit includes a bearing member configured to bear a color image,
andwherein the control unit sets the conveying speed of the recording
medium by the paper feed unit and the conveying unit so that a leading
edge of the color image on the bearing member is synchronized with a
leading edge of the recording medium.

9. The image forming apparatus according to claim 1,wherein the storage
unit is detachably loaded in the image forming apparatus,wherein the
image forming apparatus further includes a loading sensor configured to
detect loading or unloading of the storage unit, andwherein the control
unit determines whether to stop the recording medium at the conveying
unit in accordance with the detection result of the loading sensor.

10. A recording-medium feeding method for an image forming apparatus
including a storage unit configured to store a recording medium, an image
forming unit configured to form an image on the recording medium, a paper
feed unit configured to supply the recording medium from the storage
unit, the recording-medium feeding method comprising:detecting a
conveyance state in which the recording medium stored in the storage unit
is supplied; andsetting a conveying speed of the next recording medium
supplied by the paper feed unit in accordance with the conveyance state.

11. The recording-medium feeding method according to claim 10,said image
forming apparatus further comprising a conveying unit configured to
convey the recording medium supplied by the paper feed unit to the image
forming unit,wherein the conveyance state includes a time period between
a time when the recording medium is supplied by the paper feed unit and a
time when the recording medium reaches the conveying unit, andwherein a
conveying speed of the recording medium by the paper feed unit and the
conveying unit is set so that a difference between the time period and a
preset period is within a preset range.

12. The recording-medium feeding method according to claim 10, further
comprising:setting the conveying speed of the recording medium by the
paper feed unit on the basis of a time period between a time when the
recording medium is supplied by the paper feed unit and a time when the
recording medium is detected by a first detecting unit.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to an image forming apparatus that
forms an image on a recording medium, and to a paper feeding device in
the image forming apparatus. More particularly, the present invention
relates to an image forming apparatus having a function of a copying
machine or a printer, a multifunction machine having a copying function
and a document reading function, a color printer that forms a color image
on a recording medium, and a recording-medium feeding method for the
color printer.

[0003]2. Description of the Related Art

[0004]In recent years, a color printer serving as an image forming
apparatus has been frequently shared by a plurality of host computers
while being connected to the host computers via a network. A typical
color-image forming apparatus is an electrophotographic color laser beam
printer that has a low operating cost and that is not prone to failure.

[0005]There is a demand for a color laser beam printer capable of forming
images on various types of recording media. For example, it is important,
in improving usability, to print on a wide variety of types of recording
media including plain paper, thin paper, thick paper, and glossy paper,
to perform precise printing on a recording medium of a specified size,
and to print at high speed.

[0006]In such a color laser beam printer, however, the quality of an
output image is sometimes deteriorated or an undesirable image is output
because of an environmental change at an installation site of the
printer, and changes and deterioration of components of the printer due
to long-term use.

[0008]More specifically, the disclosed image forming apparatus includes a
means for detecting the leading edge of an image formed on an
intermediate transfer belt, a means for detecting a leading edge of a
recording medium placed between a registration roller and a transfer
roller, and a means for controlling the speed of the registration roller.
The recording medium is conveyed to the transfer roller, while the
registration roller controls the conveying speed of the recording medium
in synchronization with the leading edge of the image.

[0010]In this technique, a recording-medium detector is placed between a
paper feed roller and a registration roller, and the speed of the paper
feed roller is controlled in accordance with the detection result of the
recording-medium detector. This can reduce the variations in time taken
for the recording medium to reach the registration roller, and can
decrease the paper feed interval. Therefore, printing can be performed on
more recording media within a fixed period of time.

[0011]In the technique disclosed in Japanese Patent Laid-Open No.
11-194561, for example, precise printing can be performed on a recording
medium of a specified size. However, this technique is not adequate for
high-speed image formation.

[0012]In contrast, Japanese Patent Laid-Open No. 2001-213537 discloses the
above-mentioned technique of increasing the image formation speed.
However, this technique does not consider variations in the conveying
speed of the recording medium and variations in the conveying path
between the paper feed roller and the registration roller due to the type
of recording medium, the environment of the apparatus, the use state of
the apparatus, and the manufacturing tolerance of the components of the
apparatus.

[0013]In particular, in a case in which the conveying speed of the
recording medium varies and the conveying path between the paper feed
roller and the registration roller varies, after the recording medium is
detected by the recording-medium detector, the time taken for the
recording medium to reach the registration roller is not fixed and is
unstable. The variation in the time of arrival at the registration roller
due to the variation in conveying speed of the recording medium is
influenced particularly by the type of recording medium and the
environment of the apparatus. When the type of recording medium is
changed, the conveyance state also changes even if a command is issued to
convey the recording medium at the same speed. Further, even when the
same type of recording medium is used, the conveyance state changes in
accordance with the environment.

[0014]In order to remove the variations in the time taken for the
recording medium to reach the registration roller, it is conceivable to
increase the feeding interval between the recording media. It is also
conceivable to increase the distance between the registration roller and
the transfer roller. However, when the feeding interval is increased, the
number of recording media conveyed per unit time decreases, and this
reduces productivity. Further, when the distance between the registration
roller and the transfer roller is increased, the conveying distance is
increased, and the size of the body of the apparatus is increased.

[0015]Although it is possible to limit the type of paper to be used, for
example, so as not to use thick paper and glossy paper, usability is
impaired in this case.

SUMMARY OF THE INVENTION

[0016]The present invention provides an image forming apparatus that can
suppress a decrease in productivity and that can precisely form images on
various types of recording media, and a recording-medium feeding method
for the image forming apparatus.

[0017]An image forming apparatus according to an aspect of the present
invention includes a storage unit configured to store a recording medium;
an image forming unit configured to form an image on the recording
medium; a paper feed unit configured to supply the recording medium from
the storage unit; a conveying unit configured to convey the recording
medium supplied by the paper feed unit to the image forming unit; and a
control unit configured to control the paper feed unit and the conveying
unit for conveying the recording medium.

[0018]In accordance with a conveyance state in which the recording medium
supplied from the storage unit by the paper feed unit is conveyed to the
conveying unit, the control unit sets a conveying speed of the next
recording medium supplied by the paper feed unit.

[0019]A recording-medium feeding method according to another aspect of the
present invention is provided for an image forming apparatus including a
storage unit configured to store a recording medium, an image forming
unit configured to form an image on the recording medium, a paper feed
unit configured to supply the recording medium from the storage unit. The
recording-medium feeding method includes the steps of detecting a
conveyance state in which the recording medium stored in the storage unit
is supplied, and controlling a conveying speed of the next recording
medium supplied by the paper feed unit in accordance with the conveyance
state.

[0020]Further features of the present invention will become apparent from
the following detailed description of exemplary embodiments with
reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 includes a partial cross-sectional view and a control block
diagram of a paper feeding device according to a first exemplary
embodiment of the present invention.

[0022]FIG. 2 is a partial cross-sectional view showing the operation of
the paper feeding device.

[0023]FIG. 3 is a plot graph showing variations in the conveying time of a
recording medium in the first exemplary embodiment.

[0024]FIG. 4 is a plot graph showing variations in the conveying time of
the recording medium in the first exemplary embodiment.

[0025]FIG. 5 is a partial cross-sectional view showing a configuration of
a paper feeding device according to a second embodiment of the present
invention.

[0026]FIG. 6 is a partial cross-sectional view showing a configuration of
a paper feeding device according to a third embodiment of the present
invention.

[0027]FIG. 7 is a cross-sectional side view conceptually showing the
entire color image forming apparatus according to the first exemplary
embodiment.

[0028]FIG. 8 is a flowchart showing an operating sequence in the first
exemplary embodiment.

[0029]FIG. 9 is a flowchart showing an operating sequence in the first
exemplary embodiment.

[0030]FIG. 10 is a partial cross-sectional view showing a configuration of
the feeding device according to the first exemplary embodiment.

DESCRIPTION OF THE EMBODIMENTS

[0031]Embodiments of the present invention will be described in detail
below with reference to the drawings.

First Exemplary Embodiment

[0032]First, the overall configuration and operation of an image forming
apparatus according to a first exemplary embodiment of the present
invention will be described with reference to FIG. 7.

[0033]FIG. 7 is a schematic cross-sectional view of a full-color image
forming apparatus according to the first exemplary embodiment. The
full-color image forming apparatus is a full-color printer that performs
image formation with toners of four colors and that includes four
image-bearing members corresponding to the four colors.

[0034]A body of the printer includes photosensitive drums 1, 2, 3, and 4
serving as image bearing members, a laser scanner unit 25 that exposes
the photosensitive drums, and developing rollers 5, 6, 7, and 8 that
develop latent images formed on the photosensitive drums. The body of the
printer also includes an intermediate transfer belt 18, a fixing film 20,
a pressure roller 21, a paper feed tray 13, and a paper feed roller 15.

[0035]Recording media M stacked in the paper feed tray 13 are separated
and supplied by a separation roller 30 and the paper feed roller 15 that
rotates in the direction of the arrow in FIG. 7. A separated recording
medium M is conveyed while being nipped between opposing registration
rollers 31 and 32, and is sent to a nip between a driving roller 16 that
drives the intermediate transfer belt 18, and a transfer roller 17.

[0036]The photosensitive drums 1, 2, 3, and 4 rotate in the direction of
the arrows in FIG. 7 (counterclockwise). Electrostatic latent images are
sequentially formed on the outer peripheral surfaces of the
photosensitive drums 1, 2, 3, and 4 by laser light emitted from the laser
scanner unit 25. The formed electrostatic latent images are developed
into toner images by the developing rollers 5, 6, 7, and 8 corresponding
to the photosensitive drums.

[0037]The toner images formed on the photosensitive drums 1, 2, 3, and 4
are then transferred onto the intermediate transfer belt 18. In order to
form a color image, yellow, magenta, cyan, and black toner images are
respectively formed on the photosensitive drums 1, 2, 3, and 4, and are
transferred onto the intermediate transfer belt 18.

[0038]Subsequently, the toner images transferred on the intermediate
transfer belt 18 are transferred onto the recording medium M conveyed to
the nip between the driving roller 16 and the transfer roller 17.

[0039]The recording medium M on which the toner images are transferred is
conveyed to a nip between the fixing film 20 and the pressure roller 21,
and the toner images are fixed on the recording medium M by heat and
pressure at the nip.

[0040]The recording medium M on which the toner images are fixed is output
into an output tray 24 by output rollers 22 and 23.

[0044]First, a description will be given of operations of the image
forming apparatus for feeding the first recording medium and forming an
image on the recording medium in response to a print command.

[0045]When the image forming apparatus receives a print command, a paper
feed clutch 39 (FIG. 5) operates, a paper feed motor 53 is driven, and
the driving is transmitted to rotate the paper feed roller 15. Then, a
cam mechanism (not shown) operates to press the separation roller 30 into
contact with the paper feed roller 15, and a return lever 36 comes out of
a conveying path. Finally, the uppermost medium M1 of the recording media
M stacked on an intermediate plate 37, which is pivotally mounted in the
paper feed tray 13 and is biased upward by a spring (not shown), is
pressed into contact with the paper feed roller 15, and is separated and
conveyed by the paper feed roller 15 and the separation roller 30.

[0046]In order to control the rotation speed of the paper feed roller 15,
a time T11 between the time when the driving force of the paper feed
motor 53 is transmitted to the paper feed roller 15 and the time when the
recording medium M1 reaches a paper sensor S1 disposed between the paper
feed roller 15 and the registration roller 31 is detected, and the
rotation speed of the paper feed motor 53 is controlled on the basis of
the detected time T11.

[0047]The rotation speed of the paper feed roller 15 is controlled so that
a time T21 taken for the recording medium M1 to be conveyed from the
paper sensor S1 to a registration sensor S2 disposed between the
registration roller 31 and the transfer roller 17 becomes equal to a
preset time T2.

[0048]The rotation speed of the paper feed roller 15 is controlled so as
to correct variation in the feeding timing of the recording medium M.
Variation in the paper feed timing is corrected on the basis of the time
between the time when paper feeding starts and the time when the
recording medium M reaches the paper sensor S1, thus improving the
conveyance accuracy.

[0049]In the first exemplary embodiment, the registration sensor S2 is
disposed at the nip between the registration rollers 31 and 32. The
position of the registration sensor S2 is not limited to the nip, and can
be changed to other positions best suited to detect the recording medium.

[0050]After the time T21 taken for the recording medium M1 to be conveyed
from the paper sensor S1 to the registration sensor S2 is detected, the
separation roller 30 is separated from the paper feed roller 15 by the
cam mechanism. Then, a recording medium stopped at the nip between the
paper feed roller 15 and the separation roller 30 is returned into the
paper feed tray 13 by the return lever 36. Subsequently, the driving of
the paper feed motor 53 is interrupted by the paper feed clutch 39, and
the paper feed roller 15 is thereby stopped. After the recording medium
M1 is conveyed by a predetermined distance by the registration roller 31,
a registration-roller driving motor 51 stops, and the registration roller
31 also stops. Consequently, the recording medium M1 is stopped between
the registration roller 31 and the transfer roller 17. This state is
shown in FIG. 2.

[0051]The distance between an image forming position on the photosensitive
drum 1 and the transfer position of the transfer roller 17 is longer than
the distance between the paper feed roller 15 and the transfer position
of the transfer roller 17. Therefore, the first recording medium after
the input of the print command is supplied prior to formation of an image
(a toner image) on the photosensitive drum 1. This can shorten the time
taken to form an image on the first recording medium, and perform image
formation without decreasing the number of recording media on which
images are formed per unit time (hereinafter referred to as a
throughput). Further, an image can be accurately formed (transferred) on
the recording medium after the recording medium is temporarily stopped.

[0052]More specifically, the recording medium is temporarily stopped while
being nipped between the registration rollers 31 and 32, and the
conveyance of the recording medium is restarted in synchronization with
the toner image on the intermediate transfer belt 18.

[0053]For that purpose, when the toner image formed on the intermediate
transfer belt 18 comes close to the nip between the transfer roller 17
and the driving roller 16, the registration-roller driving motor 51
starts to rotate the registration roller 31.

[0054]The start timing of the registration-roller driving motor 51 is
determined so that the leading edges of the toner image on the
intermediate transfer belt 18 and the fed recording medium M1 coincide
with each other at the nip between the transfer roller 17 and the belt
driving roller 16.

[0055]A detailed description will now be given of operations of the image
forming apparatus and the paper feeding device for making the second to
last prints.

[0056]After the trailing edge of the recording medium M1 passes the paper
feed sensor S1, the driving of the paper feed motor 53 is transmitted to
the paper feed roller 15 by the paper feed clutch 39, and the paper feed
roller 15 is rotated. A recording medium Mn (n is an integer of two or
more) placed under the recording medium M1 is separated and conveyed, in
a manner similar to that for the recording medium M1. A time T1n when the
recording medium Mn reaches the paper sensor S1 is detected.

[0057]The rotation speed of the paper feed roller 15 is controlled so that
the time taken for the recording medium Mn to be conveyed from the paper
sensor S1 to the registration sensor S2 disposed between the registration
roller 31 and the transfer roller 17 becomes equal to the preset time T2.

[0058]To this control, a difference between the time T21 taken for the
recording medium M1 to be conveyed from the paper sensor S1 to the
registration sensor S2, and the preset time T2 is fed back.

[0059]That is, when T21 is less than T2, the rotation speed of the paper
feed motor 53 is set so that the rotation speed for the recording medium
Mn is lower than the rotation speed for the recording medium M1.
Conversely, when T21 is more than T2, the rotation speed of the paper
feed motor 53 is set so that the rotation speed for the recording medium
Mn is higher than the rotation speed for the recording medium M1.

[0060]When the registration sensor S2 detects the arrival of the recording
medium Mn, the rotation speed of the registration roller 31 is controlled
without stopping the registration-roller driving motor 51. The rotation
speed of the registration roller 31 is controlled so that the leading
ends of the toner image on the intermediate transfer belt 18 and the
recording medium Mn coincide with each other at the nip between the
transfer roller 17 and the belt driving roller 16.

[0061]In the first exemplary embodiment, the following means is adopted in
order to calculate the feeding amount of the toner image without any
influence of expansion and contraction of the intermediate transfer belt
18.

[0062]The intermediate transfer belt 18 is biased by a belt tensioning
spring 34 so as to have a constant tension. A leading end of a seal stuck
on the intermediate transfer belt 18 in the width direction is detected
by an optical sensor S3 that moves together with a belt tension roller 33
movable in the horizontal direction in FIG. 7. The seal has a reflectance
different from that of a portion provided outside an image forming region
on a surface of the intermediate transfer belt 18 on which the toner
image is transferred. The length of the intermediate transfer belt 18 can
be calculated from the interval at which the leading end of the seal is
detected.

[0063]Since the positions of the photosensitive drums 1, 2, 3, and 4, the
transfer roller 17, and the belt driving roller 16 are known, the amount
by which the toner image is conveyed (moving distance) can be calculated
on the basis of the emitting timing of laser light from the laser scanner
unit 25 onto the photosensitive drums 1, 2, 3, and 4.

[0064]The leading end of the toner image can be detected by an optical
sensor that detects the difference in reflectance between the surface of
the intermediate transfer belt 18 and the toner image. In this case, the
optical sensor is placed at a constant distance from the nip between the
transfer roller 17 and the belt driving roller 16. The constant distance
is more than the distance from the nip between the transfer roller 17 and
the belt driving roller 16 to the nip between the registration rollers 31
and 32.

[0065]The time taken for the first recording medium to move from the paper
sensor S1 to the registration sensor S2 is detected, and is fed back to
control the speed at which the second and subsequent recording media are
conveyed by the paper feed roller 15. This increases the accuracy in
controlling the conveyance of the recording medium M by the paper feed
roller 15, and allows the recording medium M to reach the registration
sensor S2 in the time closer to the preset time T2.

[0066]The conveying speed of the paper feed roller 15 varies for a
plurality of reasons, for example, a change in roller diameter due to
wear, a decrease in coefficient of friction, the environment of the
apparatus, changes in the coefficient of friction with the paper feed
roller 15 and in the conveying path in accordance with the type of
recording medium, and an error in diameter of the paper feed roller 15
due to production errors of components. These reasons can be reduced by
exerting the above-described feedback control. That is, the variation in
the conveyance state of the recording medium can be corrected by the
feedback control.

[0067]FIG. 3 shows examples of results of conveying-speed control exerted
on two types of recording media according to the first exemplary
embodiment.

[0068]In each type of recording medium, the conveying speeds of the second
and subsequent recording media are changed, depending on whether the time
T21 taken for the first recording medium to be conveyed from the paper
sensor S1 to the registration sensor S2 is more than or less than the
preset time T2. The time T2n taken for the n-th recording medium to be
conveyed from the paper sensor S1 to the registration sensor S2 is
controlled to be close to the preset time T2.

[0069]In the first exemplary embodiment, the rotation speed of the
registration roller 31 can be controlled so that the time T2n is within a
range ΔT2. This range ΔT2 refers to a range in which the
rotation speed of the registration roller 31 can be adjusted so as to
synchronize the recording medium with the toner image on the intermediate
transfer belt 18. That is, when the difference between T2n and T2 is
outside the range ΔT2, synchronization between the toner image and
the recording medium cannot be achieved even by adjusting the rotation
speed of the registration roller 31.

[0070]In the above-described procedure, the first recording medium
supplied after the input of the print command is stopped temporarily.
However, if the conveyance accuracy of the paper feed roller 15 and the
separation roller 30 is within the predetermined range (within
ΔT2), the sequence for temporarily stopping the first recording
medium is unnecessary. In this case, control can be exerted without
temporarily stopping the first recording medium, and the operating
sequence for the second and subsequent recording media can be performed
in the same manner. This can simplify the operating sequence of the image
forming apparatus.

[0071]In the first exemplary embodiment, however, the first recording
medium is temporarily stopped for precise image formation on the first
recording medium. Particularly when the conveying path from the paper
feed position to the transfer position is short, if image formation is
performed without stopping the conveyance of the recording medium, it is
difficult to register the recording medium with the image. Therefore, it
is essential to feed the first recording medium earlier, temporarily stop
the recording medium, and then convey the recording medium again.

[0072]FIGS. 8 and 9 are flowcharts showing operating sequences in the
first exemplary embodiment.

[0073]These operating sequences are carried out by controlling the
operations of the paper feed clutch 39, the paper feed motor 53, and the
registration-roller driving motor 51 by a controller 50 shown in FIG. 1.
As shown in FIG. 1, the controller 50 monitors signals output from the
sensors S1 and S2. The controller 50 includes a ROM 502 that stores
programs for the operating sequences shown in FIGS. 8 and 9, a RAM 503
that temporarily stores and holds data, and a timer 504 that counts the
time. The programs in the ROM 502 are read out by a CPU 501, and the
following sequences are controlled with reference to the data in the RAM
503.

[0074]First, with reference to FIG. 8, a description will be given of an
operating sequence in which the first recording medium after the input of
a paper feed command is conveyed.

[0075]In response to a paper feed command, an operating sequence for the
first recording medium starts (Step S10).

[0076]Then, the feeding speed of the recording medium is set at V1 (Step
S12).

[0078]Time counting with a timer 1 starts at the time when the paper feed
motor 53 starts to operate (Step S14).

[0079]It is determined whether the leading edge of the recording medium
has been detected by the paper sensor S1 (Step S15).

[0080]When the leading edge of the recording medium has been detected, the
time counting with the timer 1 is finished, and time counting with a
timer 2 is started (Step S16). In contrast, when the leading edge has not
been detected, the time counting with the timer 1 is continued until the
leading edge is detected. The time counting of the timers 1 and 2 is
carried out by the timer 504 in the controller 50.

[0081]On the basis of a count value T11 of the timer 1, the rotation speed
of the paper feed motor 53 is controlled. The rotation speed of the paper
feed motor 53 is switched while comparing the count value T11 with the
preset time T1. When T11 is more than T1, the rotation speed of the paper
feed motor 53 is set to be higher than V1. In contrast, when T11 is less
than T1, the rotation speed of the paper feed motor 53 is set to be lower
than V1.

[0082]Then, it is determined whether the sensor S2 has detected the
leading edge of the recording medium. When the leading edge has been
detected, conveyance of the recording medium is stopped temporarily (Step
S19). When the leading end has not been detected, the controller 50
monitors the output from the sensor S2 until the leading edge is
detected.

[0083]Conveyance of the recording medium is temporarily stopped, and
counting with the timer 2 is finished. Data on the difference between the
count value T21 of the timer 2 and the preset value T2 is calculated, and
is stored and held in the RAM 503 of the controller 50 (Step S21).

[0084]Subsequently, conveyance of the recording medium is restarted in
synchronization with movement of the toner image formed on the
intermediate transfer belt 18, and the toner image is transferred onto
the recording medium (Step S22).

[0085]Then, the above-described sequence for feeding the first recording
medium is completed (Step S23).

[0086]Referring to FIG. 9, a description will be given of an operating
sequence in which the second and subsequent recording media after the
input of the paper feed command are conveyed.

[0090]Time counting with the timer 1 is started at the time when the paper
feed motor 53 starts (Step S104). The time counting with the timer 1 is
carried out by the timer 504 provided in the controller 50. The operation
of the timer 504 is reset each time a recording medium is supplied.

[0091]It is determined whether the paper sensor S1 has detected the
leading edge of the recording medium (Step S105).

[0092]When the leading edge of the recording medium has been detected by
the paper sensor S1, time counting with the timer 1 is finished, and time
counting with a timer 2 is started (Step S106). Time counting is
performed with the timer 504 in the controller 50. In contrast, when the
leading edge has not been detected, the controller 50 monitors the output
from the sensor S1 until the leading edge is detected.

[0093]On the basis of a count value T1n (n is an integer of 2 or more) of
the timer 1, the rotation speed of the paper feed motor 53 is controlled
(Step S107). This control is exerted similarly to Step S17 in FIG. 8.

[0094]Then, it is determined whether the leading edge of the recording
medium has been detected by the registration sensor S2 (Step S108). When
the leading edge has not been detected, the controller 50 monitors the
output from the sensor S2 until the leading edge is detected.

[0095]After the leading edge of the recording medium is detected by the
registration sensor S2, the count value T21 of the timer 2 for the first
recording medium is compared with the preset value T2 (Step S109).

[0096]When T21 is more than T2, the driving speed of the paper feed motor
53 is set at a speed V3 higher than V1, the speed of the
registration-roller driving motor 51 is also set at V3, and the recording
medium is then conveyed toward the transfer position. When T21 is less
than T2, the driving speed of the paper feed motor 53 is set at V2 lower
than V1, the speed of the registration-roller driving motor 51 is also
set at V2, and the recording medium is then conveyed toward the transfer
position.

[0097]The speed V2 or V3 to which the speed V1 is switched is determined
by the difference between the count value and the preset value T2. For
example, when the count value is T21, a difference α is obtained by
subtracting T2 from T21. When L represents the distance between the
registration sensor S2 and the transfer position (see FIG. 10), V2 equals
L/(T-α) and V3 equals L/(T+α).

[0098]Herein, T presents the period between the time when the leading edge
of the recording medium is detected by the registration sensor S2 and the
time when the leading edge reaches the transfer position under an ideal
conveying condition where the recording medium is conveyed at the speed
V1 so as to move from the sensor S1 to the sensor S2 in the preset time
T2.

[0099]By the conveying-speed control adopted in the first exemplary
embodiment, the recording medium can be precisely conveyed even when the
distance between the registration roller 31 and the transfer roller 17 is
short and the adjustable speed range of the registration roller 31 is
narrow. This can decrease the distance between toner images formed on the
intermediate transfer belt 18. Therefore, the throughput can be enhanced
without increasing the size of the apparatus.

[0100]Further, when the conveying-speed control for the n-th number
recording medium is exerted by using the times taken for the first to
n-1-th prints to reach the registration sensor S2, the time T2n taken for
the n-th recording medium to move from the paper sensor S1 to the
registration sensor S2 can be made closer to T2 as printing proceeds, as
shown in FIG. 4.

[0101]By exerting the above-described speed control according to the first
exemplary embodiment, image formation on various types of recording media
can be precisely performed without increasing the size of the apparatus
and reducing productivity of the apparatus.

Second Exemplary Embodiment

[0102]The second exemplary embodiment is characterized in an operation
performed when a recording-medium sensor detects that a recording medium
is not provided in the paper feed tray 13 of the paper feeding device of
the first exemplary embodiment.

[0103]FIG. 5 is a partial cross-sectional view showing a state in which a
recording-medium sensor for detecting the presence or absence of a
recording medium is provided in a paper feed tray 13 of a paper feeding
device similar to that adopted in the first exemplary embodiment.

[0104]The recording-medium sensor includes a lever member 39 that
protrudes upward from below an intermediate plate 37 and is pivotally
supported by the paper feed tray 13, and a photointerrupter 40 that is
disposed in the main body of the paper feeding device and is shielded
from light by the lever member 39 when a recording medium is not placed
on the intermediate plate 37.

[0105]Since the recording-medium sensor is provided, the absence of a
recording medium in the paper feed tray 13 is detected even during
printing. When recording media are supplied in the paper feed tray 13 and
are fed therefrom, detection for the first recording medium is performed.
The detection result can be fed back to feeding control of subsequent
recording media.

Third Exemplary Embodiment

[0106]The third exemplary embodiment is characterized in an operation
performed when a loading sensor detects that a paper feed tray 13 of a
paper feeding device is detached.

[0107]FIG. 6 schematically shows a structure in which a paper feed tray 13
similar to that in the first exemplary embodiment is detachably loaded in
a body of a color-image forming apparatus, and shows a state in which a
loading sensor is provided to detect whether the paper feed tray 13 is
loaded or unloaded.

[0108]In the third exemplary embodiment, the paper feed tray 13 can be
drawn out of the body of the apparatus by being pulled rightward in the
horizontal direction in FIG. 6, thus allowing recording media to be
supplied therein. FIG. 6 shows a state in which the paper feed tray 13 is
loaded in the body of the apparatus.

[0109]The loading sensor includes a lever member 42 fixed to the paper
feed tray 13, and a photointerrupter 43 that is disposed in the paper
feeding device and that is shielded from light by the lever member 42
when the paper feed tray 13 is loaded in the color-image forming
apparatus.

[0110]Since the loading sensor is provided, it is possible to detect even
during a job that the paper feed tray 13 is detached and to temporarily
stop a paper feeding operation. Even when the paper feed tray 13 is
loaded after recording media are supplied therein, when the recording
media are fed, a detecting operation for the first recording medium is
performed. The result of detection can be fed back to feeding control of
subsequent recording media.

[0111]The above-described operation and control can also be applied to
cases other than the detection of the absence of the recording medium and
the detection of detachment of the paper feed tray described in the
second and third exemplary embodiments. For example, when printing is
stopped during a job because of conveying failure such as a paper jam or
a conveyance delay, the above-described detecting operation is carried
out again when feeding the first recording medium after stopping, and the
result of detection is fed back to feeding control of subsequent
recording media. This allows image formation to be precisely achieved
even when conveyance failure occurs.

[0112]Further, even when the type of stacked recording media changes, a
high conveyance performance can be maintained by performing the
above-described detection and feedback control.

[0113]While the present invention has been described with reference to
exemplary embodiments, it is to be understood that the invention is not
limited to the disclosed exemplary embodiments. The scope of the
following claims is to be accorded the broadest interpretation so as to
encompass all modifications, equivalent structures and functions.

[0114]This application claims the benefit of Japanese Application No.
2006-336010 filed Dec. 13, 2006, which is hereby incorporated by
reference herein in its entirety.